A well-known and effective transmissometer is the Dynatron 1100M, a product of Lear Siegler Measurement Controls Corporation of Englewood, Colorado. It is a dual beam transmissometer which measures light transmittance through smoke and dust. It consists of two units: an optical transceiver mounted on one side of a stack or duct and a passive reflector mounted on the opposite side. Both the transceiver and the reflector are sealed to minimize condensation or deposits of dirt on optical surfaces.
The transceiver houses a light source, photodetector and glass fiber optics. A multiple lens condenser system assures uniform light radiation through the transceiver window for high accuracy. For calibration purposes, a multi-lamp calibration system is used wherein a first lamp is used to set the zero calibration and a second lamp for the span calibration. Periodically, the instrument is recalibrated to compensate for soiling of the window during use. Thus, the Dynatron 1100M uses three light sources. One lamp for the primary measurement, another for zero calibration and a third for span calibration. Both a reference and measurement detector are used to establish the appropriate measurements and short fiber optic cables are used to direct light from the applicable source of light to the detector. It can be seen that as the three lamps age, the amount of light output from each will decrease at different rates thereby affecting the accuracy of the readings obtained.